As boards get denser, features get smaller and the number of layers increases, PCb designs come to resemble the integrated circuit chips of 10 years ago. It is not surprising that board designers, much like IC designers, need to review check plots of their design prior to sending them out for photoplotting.

It is not trivial to get paper plots, especially when the designs are large and many plots are needed in a short time. Most PCB applications support direct plotting via HPGL (Hewlett Packard Graphics Language) a well-supported vector plot format. HPGL is fine when the amount of plot data is small, but plotting large complex boards using HPGL becomes ineffective or even impossible.

HPGL: Too slow for large plots

HPGL's limited number of pen diameters forces applications to "paint" pads and traces with multiple strokes resulting in massive plot files. A large dense motherboard layer can easily produce an HPGL plot file of 10, 20 or even
50 Mbytes! Not only does it take a long time to write this amount of data
to disk but moving it to the plotter can bog down your network.

Modern plotters are really raster devices - their internal CPU and memory converts the HPGL vector data into raster data needed to drive the printhead. The plotter's built in CPU/memory were not designed to handle the large files produced by dense boards. Either no page is output, or the plotter takes hours to rasterize a single large plot.

Breaking through HPGL Plotting Limitations

One way to break through the plotting bottleneck is to use a program designed to plot very large files. GBRPLOT converts the Gerber data used to make board artwork directly into raster plot data, for a variety of printers and plotters.

Every PCB design software writes Gerber - if it didn't do a good job of creating Gerber, it couldn't be used, since board artwork is the final product of each and every design program.

GBRLPOT rasterizes the Gerber data directly on a high power workstation bypassing HPGL completely, and addresses the plotters internal raster format. The plotters CPU is responsible only for decompressing the raster data on the fly as the print head moves across the page. Even very large Gerber files rasterize in a few minutes on a UNIX workstation.

Plotting at Apple Computer

Although GBRPLOT began as a program intended to be executed by each user, one of our customers, Apple Computer's PCB CAD group, desired a networked solution. Apple's designers are busy people and wanted to plot their data to a plot directory and pick the page off the plotter.

Apple has 18 PCB designers using Zuken-Redac's Visula PCB software. They had been check plotting from within the Visula program by using a Versatec raster driver driving an old Calcomp, modified to support Versatec's Greensheet format.

The system administrator for the PCB group, Jim Langolf, asked Artwork to come see if we could help update their plotting operation.

Apple's plotting requirements included:

Checkplots generated directly from the Gerber data file used for board artwork insuring that inconsistencies between the design database and the Gerber output will be clearly visible. Some boards should be plotted at x2 scale to make problems more obvious. Final plotted images could easily exceed 24 inches.

Support for Calcomp's large format thermal plotters. These plotters can produce an E-sized monochrome page in about one minute, and are very low maintenance; no toner or ink is used.

Support for Postscript laser printers connected to the network.

The software should run on the plot server - in Apple's case, an HP 700 workstation.

The designers should not have to learn how to use a new program. They want to send the Gerber file to a spool directory and get the plot.

Meeting Apple's Plotting Needs

To meet Apple's network plotting requirements, Artwork developed a spooling program designed to handle Gerber data. GBRMON is a background program that monitors a number of spool directories and detects the presence of a Gerber file.

Once it detects a Gerber file in the spool directory, it launches GBRPLOT using the configuration file associated with that particular directory. The designer chooses the plotter and scale factor by sending his file to a particular spool directory.

Jim Langolf set up several spool directories on the server: two for the E-size Calcomp 600, one for the D-size Calcomp, and one for a B-sized Postscript printer. He also wrote a plotting script for each designer.

For example, if a designer sends a Gerber file to the directory called ex2, then GBRMON launches GBRPLOT, telling the plotting program to output CCRF, use a scale factor of 2, and send the data to the E-size Calcomp plotter.

If the designer sends the file to a directory called cool, which was setup for a Postscript plotter, GBRMON starts up GBRPLOT and tells it to output Postscript, fit the image to page, and send the postscript file to the spool directory for cool.

More Complicated Plotting Installations

Apple's installation is straightforward because the company uses a single aperture list that does not vary from project to project.
Not every design site uses a fixed aperture list. In many cases the Gerber file to be plotted is accompanied by a unique aperture list. TO support this type of installation we added some on-the-fly aperture list translators. The GBRMON calls the appropriate aperture translator, and then executes GBRPLOT.
A simple shell script prompts the designer to enter both the aperture file names and the name of the Gerber file to plot.

Summary

The GBRPLOT software installed at Apple, running on a HP 700 workstation enables 18 designers running Visual to get their check plots quickly, even for a very dense computer motherboard. A single plot server supports three different plotters - two large format thermals, and a B-size Postscript laser. There was no new software to load on the designers' workstation, and the designers did not have to learn how to use any new programs.